Crystal structure of the outer membrane active transporter FepA from Escherichia coli

Nat Struct Biol. 1999 Jan;6(1):56-63. doi: 10.1038/4931.


Integral outer membrane receptors for iron chelates and vitamin B12 carry out specific ligand transport against a concentration gradient. Energy for active transport is obtained from the proton-motive force of the inner membrane through physical interaction with TonB-ExbB-ExbD, an inner membrane complex. Here we report the crystal structure of an active transport, outer membrane receptor at 2.4 A resolution. Two distinct functional domains are revealed: (i) a 22-stranded beta-barrel that spans the outer membrane and contains large extracellular loops which appear to function in ligand binding; and (ii) a globular N-terminal domain that folds into the barrel pore, inhibiting access to the periplasm and contributing two additional loops for potential ligand binding. These loops could provide a signaling pathway between the processes of ligand recognition and TonB-mediated transport. The blockage of the pore suggests that the N-terminal domain must undergo a conformational rearrangement to allow ligand transport into the periplasm.

Publication types

  • Research Support, Non-U.S. Gov't
  • Research Support, U.S. Gov't, P.H.S.

MeSH terms

  • Amino Acid Sequence
  • Bacterial Outer Membrane Proteins / chemistry
  • Carrier Proteins / chemistry*
  • Crystallography, X-Ray
  • Escherichia coli / chemistry*
  • Molecular Sequence Data
  • Protein Conformation*
  • Receptors, Cell Surface*


  • Bacterial Outer Membrane Proteins
  • Carrier Proteins
  • Receptors, Cell Surface
  • enterobactin receptor

Associated data

  • PDB/1FEP